Work-related musculoskeletal injuries have been linked to movement repetition, improper postures, and muscle fatigue. Muscle fatigue and/or the muscle imbalances that result from fatigue may be important as direct causes and/or intermediary factors in injury development. To develop appropriate prescriptions for preventing injuries and effective strategies for rehabilitation, it is critical to understand how fatigue muscle and muscle imbalances affect the control of goal-directed movements. Thirty healthy young adults will perform an upper extremity low-load continuous work task similar to sawing for 5 minutes in time with a metronome. They will then perform either of two fatigue protocols to induce either widespread fatigue (more sawing with increased resistance) or localized muscle fatigue (repetitive shoulder flexion). They will then perform another 5 minutes of low-load sawing. Between each segment of the protocol, rates of perceived exertion and maximum voluntary contractions (MVCs) will be recorded. Kinematics, handle forces, and muscle activity (EMG) will be recorded continuously during all sawing trials. Proper movement timing is critical to many repetitive tasks. The PIs will apply novel methods to decompose the variability in the primary spatial-temporal task variables into new variables that directly affect achieving the task goal and those that do not. They will quantify both the variability and cycle-to-cycle temporal correlation structure of each resulting time series to determine how humans control movement timing during redundant goal-directed movements and how this control is altered with either widespread or localized muscle fatigue. Controlling movement stability during repetitive tasks is also critical for minimizing risk of injury. However, how muscle fatigue and/or muscle imbalances affect the control of movement stability is not well understood. The PIs will apply innovative analysis methods, developed by their lab, to directly quantify each subject's innate sensitivity to small perturbations during the continuous sawing task trials. These analyses will allow them to determine how both widespread vs. localized muscle fatigue affect the control of movement stability. This R03 project will yield vital new insights into how muscle fatigue affects task performance, about the neuromuscular and biomechanical strategies humans use to achieve this performance, and about the underlying control policies subjects adopt. Answering these fundamental questions is critical for understanding the mechanisms of musculoskeletal injury development and for developing effective rehabilitation strategies for treating patients with these injuries. Upper extremity musculoskeletal injuries (not including low back pain) are a significant and costly health care problem affecting over 375,000 people in the work-place each year. Muscle fatigue and muscle imbalances are significant contributors to these injuries. Therefore, it is important to better understand how muscle fatigue and muscle imbalances caused by localized muscle fatigue affect how humans control the timing and dynamic stability of repetitive movements.